The present invention relates to a data transmitting method suitable for a system that transmits music data through a satellite broadcast.
Digital satellite broadcast services have become common. In digital satellite broadcast services, signals are transmitted in higher quality than conventional analog broadcasts. In addition, since the frequency efficiency of digital broadcasts is high, many programs can be broadcast on many channels. For example, in digital satellite broadcast services, one satellite can provide several hundred channels. Thus, digital satellite broadcast services provide their subscribers with special channels for sports, movies, music, news, and so forth. These services broadcast special programs 24 hours a day. Among the special channels, music channels are among the more popular channels. On a music channel, a new song release guide and a hit chart can be broadcast using both video and audio data.
When the subscriber listens to a music program on a music channel, he or she may want to buy a compact disc (CD) of a song that is being broadcast. In such a case, if the subscriber could download music data of the song from the music channel, it would be very convenient. To satisfy this need, the assignee of the present invention has proposed a system that broadcasts a music program as digital audio data through a satellite, circulates the digital audio data corresponding to the music program, and allows the subscriber to purchase the music data.
When music data is broadcast and circulated through a satellite, if audio data is directly transmitted, the amount of data is extremely large and, therefore, the time for transmission is long. To solve this problem, the audio data can be compressed. An example of audio data compressing is the ATRAC (Adaptive Transform Acoustic Coding) technique. This technique has been used for compressing and recording audio data for an MD (Mini Disc). When audio data is compressed in the ATRAC format, the transmission rate for the music data can be decreased. In addition, circulated music data can be directly recorded on an MD.
Data in the ATRAC format is segmented into basic elements, each of which consists of 424 bytes of data. Each basic element is referred to as a sound group or a sound frame. One sound group is one record element. If ATRAC data is broadcasted and circulated through a satellite, it would be preferable if the data could be transmitted in such a manner that the sound groups are not broken up.
In the ATRAC format, audio data is digitized at a sampling frequency of 44.1 kHz with 16 quantizing bits. The resultant audio data is extracted with a time window of 11.61 msec. The extracted audio data is compressed to ⅕ of the original data with modified DCT (Discrete Cosine Transform).
When audio data that has been digitized at a sampling frequency of 44.1 kHz with 16 quantizing bits is extracted with a time window of 11.61 msec, the number of samples is 512. Thus, since there are 8 bits in each byte of data, the amount of audio data in the time window of 11.61 msec is:
512xc3x972=1024 bytes.
The total of the data amount on the left and right channels becomes:
1024xc3x972=2048 bytes
In the ATRAC format, with modified DCT data, data of 2048 bytes is compressed to data of 424 bytes. As indicated above, the ATRAC data of 424 bytes is referred to as a sound group or a sound frame, and one sound group is one record element of compressed audio data in the ATRAC format. Since data of 2048 bytes is compressed to data of 424 bytes, the compressing ratio using the ATRAC technique is around ⅕.
In the ATRAC format, a sound group of 424 bytes, equivalent to audio compressed data in a period of 11.61 msec, is one element of audio compressed data. When ATRAC audio data is transmitted, sound groups should be kept intact.
In a satellite broadcast, data is transmitted in the MPEG format (Moving Picture Experts Group). In the MPEG format, video data, audio data, and other data are placed on fixed length packets (188 bytes each). These packets are referred to as transport packets (TS packets). The packets are multiplexed on the same stream. Thus, when ATRAC audio data is transmitted with an MPEG stream, the ATRAC audio data must be placed on a TS packet of 188 bytes.
However, there is no relation between 424 bytes of one ATRAC sound group and 188 bytes of one TS packet. Thus, when ATRAC data is simply assigned to TS packets and the resultant data is transmitted, the sound groups are broken up. Consequently, it becomes difficult to perform an ATRAC demodulating process and an ATRAC recording process.
Therefore, an object of the present invention is to provide a data transmitting method for effectively transmitting data on an MPEG stream with PES (Packetized Elementary Stream) packets while basic elements of the underlying data are kept intact, each basic element having a predetermined data amount.
Another object of the present invention is to allow a receiving apparatus or a storage device that downloads data to check an error in transmission so as to verify the reliability of the transmission data.
To solve the above-described problem, the present invention provides a method for transmitting data (transmission information) in elementary stream packets (PES packets). The data comprises a plurality of data segments (basic elements), and each of the data segments contains a predetermined quantity of data. The method comprises forming each elementary stream packet with an integer number of transport stream packets (TS packets), and forming each transport stream packet with a quantity of the data such that an integer number of data segments is contained in each elementary stream packet. The data preferably is compressed audio data, and one data segment is preferably one sound frame or one sound group.
In accordance with the present invention, when ATRAC data is transmitted in PES packets, 159 bytes of the ATRAC data are placed in each TS packet, and one PES packet is formed with eight (8) TS packets. Thus, with one PES packet, three (3) sound frames of ATRAC data are transmitted. Consequently, one PES packet is formed with an integer number (eight) of TS packets and an integer number (three) of sound frames of ATRAC data. Since an integer number of sound frames is transmitted with each PES packet, the integrity of sound frames and PES packets is preserved.
In a preferred embodiment, the transmission information within each TS packet includes an error detection code or an error correction code. Thus, a receiving apparatus or a storage device can check for an error in the transmission information.
In the data transmitting method according to the present invention, unique information with respect to the transmission information preferably is placed in each TS packet. This unique information may contain, e.g., information representing that the current transmission data is the first PES packet, and information representing that the current transmission data is the last PES packet. Thus, the record start position and the record end position can be easily detected.
In addition, the unique information may include information representing that the current transmission data is at a particular TS packet in the PES packet. Thus, it can be determined whether data has been lost. Corresponding to the determined result, an error process can be performed.